I already bought new speakers. ADS 334IM 4" coax/component. It's a 4" coax, but it has a full sized externally mounted crossover. Probably the baddest ass 4" coax ever made. I was lucky to find some new-old stock online. I don't think they've made the 4" version in several years.
Here's a pic of one of the new speakers side-by-side with the old 3" tweeter thingy that was already in there.
To get them to fit, I of course had to enlarge the hole with a razor knife. But to get the depth, I had to take a small torch to the blower vent ducting below it, gently melt it (without it catching fire), and press it down a flat spot into it about 3" in diameter and an inch or so deep.